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From Manu Zhang <>
Subject Re: What does ReadRepair exactly do?
Date Mon, 22 Oct 2012 15:45:33 GMT
Is it through filter.collateColumns(resolved, iters, Integer.MIN_VALUE) and
then MergeIterator.get(toCollate, fcomp, reducer) but I don't know what
happens hereafter? How is reconcile exactly been called?

On Mon, Oct 22, 2012 at 6:49 AM, aaron morton <>wrote:

> There are two processes in cassandra that trigger Read Repair like
> behaviour.
> During a DigestMismatchException is raised if the responses from the
> replicas do not match. In this case another read is run that involves
> reading all the data. This is the CL level agreement kicking in.
> The other "Read Repair" is the one controlled by the "read_repair_chance".
> When RR is active on a request ALL up replicas are involved in the read.
> When RR is not active only CL replicas are involved. When test for CL
> agreement occurs synchronously to the request; the RR check
> waits asynchronously to the request for all nodes in the request to return.
> It then checks for consistency and repairs differences.
> From looking at the source code, I do not understand how this set is built
> and I do not understand how the reconciliation is executed.
> When a DigestMismatch is detected a read is run using RepairCallback. The
> callback will call the RowRepairResolver.resolve() when enough responses
> have been collected.
> resolveSuperset() picks one response to the baseline, and then calls
> delete() to apply row level deletes from the other responses
> (ColumnFamily's). It collects the other CF's into an iterator with a filter
> that returns all columns. The columns are then applied to the baseline CF
> which may result in reconcile() being called.
> reconcile() is used when a AbstractColumnContainer has two versions of a
> column and it wants to only have one.
> RowRepairResolve.scheduleRepairs() works out the delta for each node by
> calling ColumnFamily.diff(). The delta is then sent to the appropriate node.
> Hope that helps.
> -----------------
> Aaron Morton
> Freelance Developer
> @aaronmorton
> On 19/10/2012, at 6:33 AM, Markus Klems <> wrote:
> Hi guys,
> I am looking through the Cassandra source code in the github trunk to
> better understand how Cassandra's fault-tolerance mechanisms work. Most
> things make sense. I am also aware of the wiki and DataStax documentation.
> However, I do not understand what read repair does in detail. The method
> RowRepairResolver.resolveSuperset(Iterable<ColumnFamily> versions) seems to
> do the trick of merging conflicting versions of column family replicas and
> builds the set of columns that need to be "repaired". From looking at the
> source code, I do not understand how this set is built and I do not
> understand how the reconciliation is executed. ReadRepair does not seem to
> trigger a Column.reconcile() to reconcile conflicting column versions on
> different servers. Does it?
> If this is not what read repair does, then: What kind of inconsistencies
> are resolved by read repair? And: How are the inconsistencies resolved?
> Could someone give me a hint?
> Thanks so much,
> -Markus

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